Loudspeaker construction



Feb. 23,1960 w. A. KAGDIS 2,926,221

LOUDSPEAKER CONSTRUCTION Filed Nov. 21, 1957 2 Sheets-Sheet 1 ,2 Fig./

mlliam A. Kagdis 1N VEN TOR.

BY 2mm WWW 5% Feb. 23, 1960 w. A. KAGDIS 2,926,221

LOUDSPEAKER CONSTRUCTION Filed Nov. 21, 1957 2 Sheets-Sheet 2 m'llr'amA. Kagdis 1N VENTOR.

United States Patent F 2,926,221 I LOUDSPEAKER CONSTRUCTION William A.Kagdis, Edgewood, Md. Application November 21, 1957, Serial No. 697,873

4 Claims. (Cl. 179-1155) This invention relates to loudspeakerconstructions and more particularly to dynamic speakers.

The most popular design of loudspeaker in use today is the dynamicspeaker which has a moving coil in a magnetic field as the motormechanism. This mechanism consists chiefly of three parts: the voicecoil, the permanent or electro-magnet and the flux return structure.

When permanent magnets are used, they are, for reasons of economy,usually cut from long cylindrical rods of magnetic material. The magnetsthus obtained are magnetized so that one end of the cylinder is thenorth pole and the other end the south pole. Associated with the magnetis the flux return structure which is made of mild steel chosen for itshigh permeability to magnetic lines of flux. It is constructed so thatthe lines of flux passing from the north to the south pole of the magnetpass through the mild steel and are forced to jump a gap wherein thevoice coil is located. The flux return structure serves to bend thelines of flux in a desired manner in order to achieve the greatest fiuxdensity in the voice coil gap.

This gap usually takes the form of a ring, the sides of which form thepoles of amagnet. The 'inner side is one pole and the outer side is theother pole. Often the cylindrical magnet itself forms the inner pole,and the flux return structure the other pole. The density per squarecentimeter of the lines of flux within the gap and the depth of the gapover which the flux density is uniform are both very importantconsiderations in good' loudspeaker design.

The third part of the mechanism, the voice coil, con-- sists of severalturns of fine wire wound, usually in two layers, on a paper or thinmetal tube. The material from which the tube is made is chosenfor itscharacteristics of rigidity and lightness. 11

When the voice coil is put in the magnetic field, at any point along thecircumference of the voice coil, the wires of the coil are essentiallyperpendicular to the direction of the lines of magnetic flux and when acurrent is allowed to flow in the coil, force is exerted on the coil tomove out of the magnetic field. The direction of this movement is in adirection perpendicular to the direction of the lines of flux i.e. axialalong the voice coil. The polarity of the magnetic field and thepolarity of the coil determines the axial direction (i.e. toward or awayfrom the magnetic field) that the coil will take.

The operation of this kind of a motor is governed by the same laws whichapplyto the operation of electric motors and differs only in that theforce is directed longitudinally rather than in a rotational direction.

As in conventional motors, the is dependent upon a. The density of themagnetic field .f.

y b., The total area over which thisfield is distributed c. The numberof turns of wire in the voice coil (wire, size, distance between turns,etc.)

total force developed 2,926,221 Patented Feb. 23, 1900 coil.

My design does not attempt to achieve an increase in flux density of asingle magnetic field. It will, however, fulfill an object of permittingthe construction of speakers with a greater total flux and thedistribution of this flux over a greater area. In addition it willfulfill an object of permitting the construction of speakers wheregreater control'over movements of the voice coils and diaphragm can beexercised by the input signal.

In general it can be stated that the total number of magnetic lines thata magnet will deliver depends upon the size of the magnet. Therefore, itis desirable touse large magnets in well designed loudspeakers in orderto achieve a large amount of flux with which to drive the voice coil.However, the flux density available in the voice coil gap is limited bythe material .used in making the flux return structure and its design.The maximum flux density presently available from permanent magnets islimited by the saturation of the mild steel flux return material. With aone inch center pole, this saturation occurs at 14,000 lines and anincrease in the size of the magnet, using the same flux returnstructure, would not increase the flux density available in the voicecoil gap. The permeability of the mild steel prevents this. The use of alarger magnet merely allows the designer to increase the area over whichthe flux is distributed. This latter point is of great merit inloudspeaker design.

One method of increasing the flux density in the voice coil gap is todecrease the size of the gap, that is, the distance between the poles atthe gap. This, however, presents grave problems in manufacture andoperation of the loudspeaker.

The following figures, taken from G. AJBriggs book tilted Loudspeakersgives an approximation of the weight of magnet castings required forvarious flux density readings on a one inch center pole and serve toillustrate the relationship between magnet size and flux densities inthe voice coil gap:

These figures do not indicate the total flux available from the magnetand do not show the area over which the total flux is distributed. Asindicated by these figures, a well designed loudspeaker using a 56 oz.magnet will have a flux density of 14,000 lines per square centimeter ofvoice coil gap. The gap itself will probably-be in the form of a ringone inch in diameter and one half inch deep. Thus, there is a limit tothe density of flux one can achieve and the maximum area over which thisdensity can be'distributed. Therefore the loudspeaker designer isrequired to compromise one factor for the other. For example, if hedesires to obtain maximum flux density, then the distance over which theflux is distributed must reach a minimum. If he desires maxi-f mumdistribution, then the flux density must be sacri- In the design Ipropose, it is possible to increase the 8' oz. magnets delivering atotal of 25,000 lines-of flux each;.will:cost less than one 1602. magnetof comparable" totalfiux. In addition, this design permits the manufacd.The amount of current passing through the voice seesaw v o ture ofloudspeakers with total fluxes much larger than hitherto feasible.Loudspeakers with magnets larger than 56 02. are quite rare since thecost of larger magnets makes their manufacture economically unsound. I

' This design is also a more convenient and more economical way ofincreasing the amount of force which is used to drive the diaphragm ordiaphragms. It is desirable that the movement of the voice coil be underdirect control at all times and that the force with which it is moved beuniform over a large distance. It has been shown by H. A. Hartley in thearticle Realistic High Fidelity in Radio and Television News, July 1956,that the movement required of various sizes of diaphragms at lowfrequencies to avoid bass distortion is as follows for a .30 cycle note:

Thus, an 8 inch speaker must have a constant fiux density over thedistance of one inch in the voice coil gap in order for it to be able toreproduce a 30 cycle note with out distortion. Such a speaker wouldrequire a large magnet in order to achieve a desired fiux density of14,000 lines per square centimeter over a voice coil gap one inch indiameter and one inch in depth. Assuming that such a speaker wereavailable, the addition of another magnet and voice coil assembly, as Ipro-pose, would permit the same diaphragm to be driven with almost twicethe force available from the single magnet speaker with a. con sequentincrease in fidelity.

4 Another feature of the proposed design lies in the distribution of theflux. With propitious placement of the voice coils, it is possible toachieve a greater distribution of the total flux.

When the voice coils are placed so so that only an equal part of eachcoil is immersed in their respective magnetic fields a push-pull actioncan be obtained. The coils are placed so that that portion which is notimmersed in the field is closer to the center of the voice coil form.Thus, regardless of which direction the coils move, when one coil moves,it pushes the other coil an equal number of turns into its magneticfield. When .the input current reverses, the second coil is able to pushthe first coil back in the direction from which it came. This actiondiifers from conventional designs in the following manner of operation:if a single voice coil and magnetic field were used, the voice coilwould be undoubtedly centered in the field (there is no obvious reasonfor placing it otherwise). When current is allowed to flow in the coil,it causes the coil to move out of its magnetic field. The initial forceis the greatest since, when it starts from rest the greatest number ofturns of the voice coil are in the field. As it begins to move, however,some turns are forced out of the field and cease to exert a moving forceon the diaphragm. .If the excursion is great and the entire coil movesout of its field, there is no restoring force and the diaphragm mustrely upon the suspensions toreturn it to its original position. Thiseffect has been a source of concern for many years and is a severe causeof distortion, particularly in the bass region of frequency responsewhere cone excursions are greatest. The design proposed above is ineifect a method of increasing the distance over which the voice coil iscontrolled by the .input signal without relying on mechanicalsuspensions to return the voice coils. It is similar to increasing thedistance over which the total flux is distributed.

In one embodiment of my invention both voice coils are placed completelyimmersed in the magnetic field. Where cone excursions are comparativelysmall, for example when cycles of 100.0 and above are fed i the totalcone excursion may be only a' small fraction of a centimeter. This voicecoil placement would then be preferred in high frequency units, that is,speakers designed to reproduce the higher frequencies.

I have built a number of speakers and they work very well. They weremade as follows: two dynamic speakers of the same design andconstruction and of the same manufacturer were glued together, onefacing the other, at the rims. Then one of the cones of one of thespeakers was cut away at the rim and at the voice coil leaving onespeaker intact and the other with merely a basket, spider, voice coilform and voice coil. Then the two voice coils were connected together bycementing a tube of balsa wood of the diameter of the voice coils and ofa length sufiicient to fit nicely into the distance between the voicecoils without displacing them from their resting position. This tubeserved to transform the two independently operating voice coils into onecontinuous tube with two voice coils on it and operating a singlediaphragm.

The voice coils of my speakers may be operated independently, in seriesor in parallel. The latter would be preferred for high fidelityapplications. When operated in parallel, their total impedance is halfthat of either of them, permitting the use of large voice coils with lowimpedance, a desirable feature in audio work.

The suspension mechanism on a loudspeaker serves two purposes: itmaintains the voice coil in the center of the magnetic gap and alsoserves to restore or return the voice coil to the rest position when itis moved out of the magnetic field. 1

In its simplest form, the suspension consists of a surround and aspider. Much attention has been given recently to the design of thesurround. It consists of the rim of the cone, which has been corrugated,and acts as a compliance between the rigid basket and the rim of themoving cone. The spider is a corrugated disc placed at the junction ofthe cone and voice coil form, which not only acts as a compliance butalso serves to center the coil in the magnetic gap.

Theoretically, the objective in designing a rim compliance or surroundis to achieve an infinitely compliant rim which would allow the cone tomove any distance without exerting a force on it. Any force which tendsto restrict the'movement of the cone enters in as distortion inplayback. If the surround were absent, the cone would tend to rotatefrom side to side using the spider as a pivot, and the voice coil wouldrub against the poles of the magnet. Without a surround, the onlysuspension on the cone would be the spider.

Thepresent design attempts to achieve minimum side oscillations of thisnatureby using two spiders, the function of one of which is to relievethe surround of all of its functions as far as centering of the voicecoil is concerned. Further, since the voice coils operate as a pushpullsystem, the surround need no longer be used to help return the voicecoil to resting position. In fact, this design permits elimination "ofthe surround entirely or at least, the use of a surround of greaterflexibility than any present day speaker design permits.

It has long been knownthat larger speakers will reproduce bass notesmore efiiciently than small ones. Where a six inch speaker may have abass resonance at around -200 cycles, fifteen inch speakers areavailable with much lower resonances-around 20 to 50 cycles. Theexplanation of this phenomenon is very complicated butthe principle isused in high quality installations by dividing therange of frequenciesto be reproduced and channeling them to speakers designed forreproducing those frequencies. For example, a three-way system used inthe home may have a 12 or 15 inch speaker reproducing frequencies from30-350 cycles, a 6 inch speaker reproducing the range from 350-3500cycles and a compression driven horn speaker reproducing those from 3500to 29,000 cycles As far as bass reproduction is-concerned, speakerslarger than 18 inches theoretically should be better able to extend thelower end of the frequency spectrum to ten cycles. Their manufacture,however, is not feasible under present designs. One reason for this isthat with large speakers, problems in suspensions arise and as thespeakers become larger, it is more difficult to design a spider andsurround which will function to keep the voice coil properly centered inthe magnetic gap. Very often, cone sizes and shapes are chosen withthese considerations in mind.

In addition, larger speakers require heavier cones and as cone massincreases, it is more and more diflicult to design into the spider andsurround the restoring force necessary to return the voice coil to theresting position. With the present design, the problem of cone mass isstill a critical one. However, since the spiders are intended merely tokeep the voice coils centered in their magnetic fields, relieving thecone of this function, greater flexibility is permitted in cone design.Cones need no longer be chosen or designed with the problem in mind ofvoice coil centering and voice coil return. In fact, it is no longernecessary for loudspeakers to have round cones, they may have flatsquare ones, or cones of any shape.

An additional feature of my design rests in the ability to constructspeakers with a plurality of motor devices to drive the same movingmembrane. For example, where large diaphragms are to be used, it ispossible to construct a speaker with two or more sets of double voicecoils acting in push-pull to move it.

Typical speaker constructions are illustrated to show variousconfigurations of speakers that typify the principles of the invention.7

Figure l is a longitudinal sectional view of a dynamic speaker that hasa single diaphragm that is driven by two voice coils .connected by arigid member.

Figure 2 is a perspective view of the two voice coils showing the rigidmember coupling them and the method of cementing the voice coils lead-inwires to the rigid member.

Figure 3 is a schematic representation showing the two voice coils inrelation to their magnets.

Figure 4 is a schematic view of a modification of the voice coilarrangement where they are extended for complete immersion in themagnetic fields produced by the magnets.

Figure 5 is a view in elevation and section showing a modification ofthe basic design of Figure 1.

Figure 6 is a sectional view of another modification of dynamic speakerwherein the baskets are nested to reduce the size and weight of thevoice coil form.

Figure 7 is a largely schematic view showing a modification of theinvention wherein a single diaphragm is actuated by a plurality of motormechanisms.

In the accompanying drawings there is a loudspeaker 10 which isconstructed in accordance with the principles of the invention. Theloudspeaker has a frame 12 made of two frame sections 14 and 16connected together at their corresponding edges by interlocked flanges18. A cushion ring 20 is adhered to one surface of the flanges 18 and isused in the ordinary way. Annular pocket 22 is at the innercircumference of the interlocked flanges 7-18 and has a pliant ring 24in it. The rim 26 of diaphragm 28 is attached, as by gluing, toring 24and a-surface of pocket 22. The diaphragm 28is in the form of a conealthough other configurations may be used. Frame 12 has the shape of twoconventional, permanent magnet speaker baskets or frames which arebolted or otherwise connectedtogether at their flanges. Openings 30 and32 in the frame sections 14 and 16 enable the diaphragm to function assuch without significant aerodynamic rsistance.

Permanent magnets 34 and 36 plus "flux return structures 38 and 40 foreach permanent magnet, are mounted in axial spaced relationship to eachother and on the seci has hollow ends within which magnets 34 and 36 arelocated. Two voice coils 46 and 48 are on the ends of coil form 44 andthey are immersed in the high density flux across gaps Sil and 52 formedbetween the magnets 34, 36 and their flux return structures 38, 40,respectively. Diaphragm neck 29 is fitted over the coil formintermediate between the voice coils 46 and 48 and is glued in place. Asillustrated in Figure l coil form 44 is capable of oscillations whoseexcursion will be limited by the size and capacity of a number of theparts. However, the excursions are produced by two voice coils, twoperma; nent magnets and two flux return structures. The only rearsonthat permanent magnets are selected for illustration in preference toelectromagnets is that permanent magnets are presently more popular indynamic speaker construction. The principles of my invention are equallyapplicable with electromagnetically energized speakers.

Voice coils 46 and 48 are operatively connected by electrical conductorsadhered to the surface of the hollow coil form 44. They are shown asbeing connected in parallel, although serial connections or individualconnections could be used. In the latter alternative, four leads will berequired instead of the illustrated pair of leads. In the sense that thediaphragm is oscillated by the movement of a voice coil in a magneticfield, my speaker operates the same as any other dynamic speaker.However, by selection of polarity of voice coils 46 and 48 and the fieldproduced at the gaps 50 and 52, my invention places the diaphragm inmechanical push-pull response to the application of a-signal voltage tothe voice coils.

One of the results achieved is the extreme flexibility with which therim compliance or surround 58 of diaphragm 28 can be constructed. Thepush-pull diaphragm motor relieves the rim compliance 58 of allrestoring functions for the diaphragm. Therefore the rim compliance canbe made much more flexible or completely eliminated since the compliancewill not be called upon to oppose oscillations of the diaphragm alongthe desired axial path of movement.

There are means carried by frame 12 for positioning the voice coils 46and 48 in gaps 50 and 52 and secondarily in some instances, to returnthe coils to the rest position after an excursion. These means relievethe rim compliance 58 of these functions making it possible to furtherincrease the flexibility of the rim compliance or even, omit the rimcompliance completely. The omission of the surround or rim compliance inspeakers capable of reproducing reasonably undistorted low frequenciesis achieved by my invention. The coil positioning means consist of apair of spiders 60 and 62 which may be of the rigid or flexible type. Ihave illustrated the flexible type and shown them as being attached toframe sections 14 and 16 by mounting rings 64and 66 on sections 14 and16 and to which the periphery of the spiders 60 and 62 are secured, asby gluing. Apertures 68 and 70 are in the spiders 6t} and 62 and havethe coil form 44 passed through them. The spiders are located betweenthe voice coils 46 and 48 and on opposite sides of neck 29 of diaphragm28.

The design of the coil forms can vary from those in Figure 1. Coil forms46a and 48a (Fig. 3) are partially immersed in the high density fluxacross gaps 50a and 52a. Extended coils 46b and 48b could be used oncoil form 44b and they would always have the same area immersed in theflux across gaps 50b and 52b during excursions of the coil form 44b.These alternatives also prevail for all other forms of the inventionwhich fall within the comprehension of the invention.

' Reference is now made to Figure where there is dynamic speaker 80. Inessence, this speaker is the same as speaker 10. There are certainstructural alterations. These, though, are concerned with mechanicaldetails. Speaker 80 has its primary function as a tweeter while speakeris primarily useful as a woofer. Frame 82 is shown as made of twosections 84 and 86 bolted together at their mating flanges 88 andsupporting a cushion ring 89 thereon. The diaphragm 90 has its neck 92glued or otherwise attached to coil form 93. The coil form has a centersection 94 made of balsa wood or some other light material (somesynthetic resins) and hollow end sections 95 and 96 which fit into fluxgaps formed by magnets and flux return structures operatively connectedwith the magnets. Voice coils 97 and 98 on coil form 93 have separateleads enabling the coils to be connected in series or parallel or toremain independent. Terminals 99 and 100 are carried by frame or basket82 and have the leads connected therewith.

In Figure 6 there is a permanent magnet loudspeaker 110 whose length isreduced by nesting the two sections 112 and 114 and joining themtogether at their peripheral flanges 116. Cushion ring 118 is adhered toone or both of these flanges so that the speaker can be mounted properlyfor correct acoustics. Diaphragm 120 is disposed between the framesections 112 and 114 and has its rim 122 interposed between flanges 116and fastened firmly in place. Rim compliance or surround 124 is adjacentto the rim 116 and is made with the great degree of flexibility that ispractically possible in view of the push-pull nature of the power inputand the guiding functions that are wholly assumed by spiders 126 and128. These spiders are secured to sections 112 and 114 of the basket orframe and they have central openings which guide the coil form 130 so asto constrain its oscillations to a single axis. Voice coils 132 and 134on the coil form 130, are, therefore, correctly guided and maintainedaligned in their gaps 136 and 138.

The permanent magnets 140 and 142 are secured to the center parts of thetwo frame sections of the speaker and they have flux return structures144 and 146 operatively connected adjacent to them in order to form thegaps 138 and 136 within which the coils 134 and 132 are operable.Speaker diaphragm 120 is secured to the coil form 130 between thespiders 128 and 126.

Figure 7 illustrates one possible configuration which uses the power ofmore than one motor mechanism, with each operating in unison to drivediaphragm 150. Coil forms 152 and 154 have voice coils 156, 158 and 160,162 thereon which are movably mounted in gaps 164, 165, 166 and 167 ofpermanent or electromagnets 170, 172, 174 and 176. The diaphragm iscemented or otherwise secured to the coil forms whereby it is driven ina push-pull fashion as are the other described modifications of theinvention.

Although the principles of the invention are applicable in connectionwith speakers other than permanent magnet dynamic speakers, for exampleelectromagnet loudspeakers, the invention makes possible the productionof permanent magnet speakers with a driving capacity that is far inexcess of the capacity capable of being achieved by a single permanentmagnet. This is due to the flux density limitations of the bestavailable and commercially competitive magnetic material. By using thedescribed motor mechanism, the total flux is doubled while maintainingthe flux density at the same maximum and this results in driving powerand response alaerity in excess of that that is achievable with a singlemagnet.

-Various changes and modifications may be made without departing fromthe scope of the following claims.

What is claimed as new is as follows:

'1. In a speaker which has a frame provided with a pair of framesections connected at their outer edges, a diaphragm, a pair of magnets,a flux return structurei slightly spaced from each magnet and coactingtherewith to form a pair of flux gaps therebetween, said magnetsandreturn structures secured to said frame sections and: located on theouter sides thereof, a coil form to which: said diaphragm is attached,said diaphragm located between said sections, two voice coils, one coilimmersed in the flux of each gap, means for applying a signal voltage toeach coil and placing said coil in mechanical pushpull between said gapsand constituting the sole meansfor oscillating said diaphragm in twocoaxial directions, a rim compliance at the edge of said diaphragm andcon-- nected with said frame, and means consisting of at least onespider operatively associated with said coil form to retrict itsoscillations to axial movement and thereby relieve said rim complianceof diaphragm motion constraining functions whereby said rim compliancemay beconstructed more flexibly.

2. In a dynamic speaker, a frame, a diaphragm, a coil form, a pair ofmagnets, a pair of flux return structures to form high flux gaps, a rimcompliance at the edge of said diaphragm, a coil form having voice coilsthereon which react with the magnetic flux across said gaps to oscillatethe coil form, said diaphragm having a portion which is secured to saidcoil form, means including a pair of spaced spiders for constraining themovement of said coil form and relieving said rim compliance of coilform constraining functions, said voice coils wound oppositely to havepolarities which coact with the flux across said gaps to push and pullsaid coil form simultaneously from opposite end portions and therebyrelieve said rim compliance of the function of restoring said diaphragmto a rest position.

3. In a loudspeaker which has a frame, a diaphragm provided with a neckand a rim compliance which is attached to said frame, push-pull meanssecured to said neck for oscillating said diaphragm in a constrainedaxial path, said push-pull means including magnets and flux returnstructures producing flux gaps that are axially aligned with said neckof said diaphragm and spaced on each side of said neck, a coil formhaving portions located in said gaps and to which said neck is secured,voice coils on said coil form and at least partially immersed in saidgaps during excursions of said coil form, means constraining themovement of said coil form to axial motion and including spaced spidersdisposed on opposite sides of said neck of said diaphragm, said framehaving two sections, one of said magnets carried by each of saidsections, said sections having flanges at their outer edges, and meansjoining said flanges together to unify said sections.

4. The loudspeaker of claim 3 wherein said sections are approximatelyconcavo-convex in configuration, and said magnets and flux returnstructures are disposed on the outer side of said sections with saiddiaphragm lo cated in the space between the inner sides of saidconcave-convex sections.

References Cited in the file of this patent UNITED STATES PATENTS1,105,924 Pridham et al. Aug. 4, 1914 1,808,149 Smith June 2, 19312,671,135 Woodward Mar. 2, 1954 2,832,843 Miessner Apr. 29, 1958 FOREIGNPATENTS 312,950 Great Britain June 4, 1929 400,169 Italy Nov. 25, 1942705,100 Great Britain Mar. 10, I954 1,102,175 France May 4, i955

